1. Field of the Invention
This invention relates to a controller and a control method for an electric power steering apparatus installed for vehicles.
2. Discussion of the Background
An electric power steering apparatus provides assist power corresponding to steering operation by an electric motor. It is possible that a voltage drop of a power source which provides power to the electric motor happens because of an abnormality of a battery, a short-circuit of power cable or a loose connection of a connector. The voltage drop affects assist performance of the electric power steering apparatus such as lack of the assist power which makes a driver feel a sense of incongruity or absence of the assist power which makes the steering operation become heavy. A source voltage monitor which always monitors the source voltage is built in a electric power steering apparatus in order to warn the voltage drop to the driver when the source voltage becomes less than a predetermined value.
Generally, the voltage drop occurs temporarily due to starting of other auxiliary equipment, because a common battery is used for both of the auxiliary equipment and the electric power steering apparatus. In addition, the temporary voltage drop occurs due to an instantaneous short-circuit of power cable or an instantaneous loose connection of a connector. Such voltage drop is just temporary, and is regained to normal voltage immediately. However, the source voltage monitor detects the voltage drop as an abnormality when the voltage decreases in less than the predetermined value, even if the voltage drop is temporary such that the assist performance is not affected.